One of the major parameters that determine the performance of a photodiode is its responsivity to light. The responsivity, R, is defined as the current generated by the photodiode (PD) per unit incident light power and is typically expressed in amperes per watt (A/W). This detection property of a PD can be equivalently described by the quantum efficiency ηext, which is 100% when every incident photon excites an electron-hole pair that is eventually collected by the electrodes. The relation between the responsivity and the quantum efficiency is   R  =            e      hv        ⁢          η      ext                      where hv is the photon energy and e is the electron charge. η is dictated entirely by the number of electron-hole pairs collected, whereas the responsivity is dependent on wavelength (photon energy).        
The purpose of this invention is to improve the intrinsic responsivity of a photodiode such as an avalanche photodetector (APD). A standard positive-intrinsic-negative (PIN) PD is quantum-limited since, for each incoming photon, a single electron-hole pair (at most) is contributed to the total photocurrent. An APD, by virtue of its internal avalanche gain, can achieve a much higher responsivity than a PIN-PD. However, the intrinsic responsivity of the APD (i.e., at unity gain) is still critical to the device performance. In particular, the sensitivity of a fiber optic receiver utilizing an APD will improve proportionally with improvements in the intrinsic responsivity, so increasing this responsivity at unity gain as much as possible is very important.
Specifically, this invention is intended to provide a structure that increases the reflectivity of the front (top) contact region of a back-illuminated, surface-normal photodiode such as an APD. The responsivity of a surface-normal APD is determined by several factors, such as the thickness of the absorption layer, the optical loss in material other than absorption layer, and reflections from the interfaces. For absorption layers of finite width, some fraction of the incident light will pass through this layer without being absorbed and may be partially absorbed in the contact material without contributing to the photocurrent. But if the contact structure is designed so that this light is efficiently reflected back for a second pass through the absorption layer, the overall fraction of the incident light contributing to photocurrent will increase.
In the past, it has been known to provide an at least partially reflecting layer within a back illuminated photodiode so that unabsorbed light that passed through a semiconductor absorption layer could be recovered by way of being reflected back to the absorption layer for a second pass rather than being lost. As telecommunications specifications become more stringent, such light recovery schemes become particularly critical to provide devices that will meet these challenging requirements. For example, a prior art search has revealed that recently issued U.S. Pat. No. 6,437,362 in the name of Suzuki assigned to Matsushita Electric Industrial Co., Ltd. (Osaka, JP) incorporated herein by reference discloses a diode structure that at least partially addresses these concerns. Suzuki discloses the use of a distributed Bragg reflector made from III-V semiconductor material where a metallic ohmic contact is made to be the reflector itself. Although the structure taught by Suzuki seems to perform its intended function, the invention disclosed herein, provides a different structure that offers significant advantages over prior art back illuminated photodiodes.
It is preferred to provide the ohmic contact directly to the PD epitaxial layer, rather than through the Bragg reflector semiconductor layers, as this impairs the conductivity. Furthermore, the formation of a distributed Bragg reflector between the epitaxial layer and the contact adds significant complexity to the manufacturing process.
It is therefore an object of the invention to provide a photodiode that efficiently recycles light that has passed through its active light absorbing material thereby increasing its responsivity to incoming light while maintaining good ohmic contact to the epitaxial layer.